1. Field of the Invention
This invention relates to a picture reproduction apparatus for reproducing video signals recorded on, for example, magnetic sheets.
2. Description of the Related Art
A conventional example of the video signal recording and reproduction apparatus is shown in FIG. 2. Now assuming that a video signal to be recorded has a spectrum shown in FIG. 3, how to record and reproduce it on and from a magnetic sheet by using a magnetic head is explained below.
In FIG. 3, a reference numeral 1 denotes the spectrum of a luminance signal to be recorded with its sink tip at f.sub.3 =6.0 MHz, and its white peak at f.sub.4 =7.5 MHz as obtained from a frequency modulator. A reference numeral 2 indicates its spectrum of a color-difference signal to be recorded which includes R-Y signal which is amplitude-modulated at center frequency f.sub.1 =1.2 MHz and B-Y signal which is amplitude-modulated at center freqnecy f.sub.2 =1.3 MHz. Here, the R-Y and B-Y signals are processed in sequence for each horizontal period, forming a line-sequential signal.
The process of recording and reproducing an NTSC signal, the video signal, will be explained, with reference to FIG. 2.
The circuit shown in FIG. 2 includes an input terminal 3, a band pass filter 4, a delay circuit 5, subtractors 6 and 7, and a delay circuit 41. The NTSC signal is separated into a color signal and a liminance signal by a "comb filter" constructed of said elements 5, 6, 7 and 41. That is, in order that the deterioration of the vertical resolution for the luminance signal in the comb filter falls within an allowable range, the NTSC signal is made to pass through the band pass filter 4 whose frequency characteristic has its center frequency of f.sub.C =3.579545 MHz (the frequency of the NTSC color subcarrier) and then through the delay circuit 5 whose delay time is equal to the horizontal period of the NTSC signal. The output signals of the filter 4 and delay circuit 5 are then applied to the subtractor 6 to remove the luminance signal whose primary component has a frequency equal to integer times the horizontal frequency f.sub.H of the NTSC signal passed through the band pass filter 4. Hence, a color signal is produced from the subtractor 6.
Meanwhile, the NTSC signal from the input terminal 3 is routed to another delay circuit 41 having the same delay time as that of the first delay circuit 5. Its output and the output of the subtractor 6 are then applied to another subtractor 7 to remove the color signal from the NTSC signal. Hence, a luminance signal is produced from the second subtractor 7.
The luminance signal from the subtractor 7 passes a low pass filter 13 and then a pre-emphasis circuit 14 by which the higher frequencies are emphasized with respect to the lower ones. After that, by frequency modulator 15 and a high pass filter 16 is obtained the spectrum of the luminance signal shown by "1" in FIG. 3 which is then applied to an adder 17. Here, the low pass filter 13 has such an output characteristic as, for example, shown in FIG. 5 that the upper sideband is substantially flat at or near the frequency f.sub.C of the NTSC color subcarrier and the cut-off frequency at 3 dB is 4.2 MHz.
The color signal from the subtractor 6 is applied to an NTSC color signal decoder 8 which then produces color difference signals, that is, R-Y and B-Y signals in parallel. Then, by a line-sequential operating and offset adding circuit 9, the R-Y and B-Y signals are serially transferred to form a line-sequential signal, and a DC offset representing which of the R-Y signal and the B-Y signal is then added. Then, the higher region is emphasized by a pre-emphasis circuit 10. By a frequency modulator 11 and a band pass filter 12, it is then changed to a color difference signal to be recorded (see "2" in FIG. 3). In the above-described circuit 9, addition of the offset is controlled in such a manner that the central frequencies of the R-Y and B-Y signals coincide with the aforesaid values of f.sub.1 and f.sub.2 respectively (see FIG. 3).
The output of the band pass filter 12 is applied to the adder 17 which then produces a video signal to be recorded whose spectrum is shown in FIG. 3.
The output signal of the adder 17 is applied to a recording amplifier 18 of which the output is supplied through a switch 19 to a magnetic head 20.
The magnetic sheet 21 is driven by a motor 22 to rotate in synchronism with the field period of the NTSC signal. As the result, the video signals for one field are recorded in each concentric recording track on the magnetic sheet 21 by the magnetic head 20.
During the time of recording on the magnetic sheet 21, the output signal (luminance signal) of the subtractor 7 is introduced to a vertical synchronizing signal separation circuit 36, so that the vertical synchronizing signal is separated. Also, when the apparatus is switched to the recording mode, a switch 39 is set so that the vertical synchronizing signal is outputted to a terminal 40. The vertical synchronizing signal appearing at the output terminal 40 is used as a reference signal for a servo circuit (not shown) controlling the rotation of the motor 22. Thus, when in the recording mode, the motor 22 is made to rotate in synchronism with the vertical synchronizing signal.
When in the reproduction mode, the video signals recorded on the magnetic sheet 21 are read out by the magnetic head 20 and the readout signals are applied through the switch 19 to a reproducing amplifier 23. The output signal of the reproducing amplifier 23 enters a high pass filter 24 where the color difference signal component is removed from the reproduced signal. After that, the output signal of the high pass filter 24 is applied to a frequency modulator 25 and therefrom further processed through a low pass filter having an almost equal characteristic to that of the low pass filter 13 and a deemphasis circuit 27 to regain the luminance signal.
Meanwhile, the output of the reproducing amplifier 23 is introduced to a band pass filter 28 to remove the recorded luminance signal component. After that, it is applied through a frequency demodulator 29 and a low pass filter 30 to a deemphasis circuit 31. Because the output signals of the deemphasis circuit 30 are line-sequential color-difference signals having offset, the offsets of the R-Y and B-Y signals are removed by a line-coincidence circuit and an offset correction circuit 32. The thus-obtained coincidence signal is then processed for rectangular double-balanced modulation by an NTSC encoder 33. Hence, an NTSC color signal is produced.
The NTSC color signal from the output of the NTSC encoder 33 and the luminance signal from the output of the deemphasis circuit 27 are added by an adder 34 to produce a reproduced NTSC signal at an output terminal 35.
Also, a quartz oscillator 37 produces a clock signal of f.sub.C =3.579545 MHz. This clock signal is applied to the NTSC encoder 33 and is also counted down by a counter 38. So, the counter 38 produces a signal of the same period as that of the vertical synchronising signal.
When a picture is being reproduced, the switch 39 passes the output of the counter 38 to the servo reference signal output terminal 40. Thus, the rotation of the motor 22 is controlled by the signal formed on the basis of the output of the quartz oscillator 37 and having the same period as that of the vertical synchronizing signal.
In such an apparatus, however, the following problem exists. The recording system has a comb filter including the delay circuit 5 and subtractor 6 as shown in FIG. 1 in order to separate the NTSC signal into the luminance signal and the color signal. But, even with the use of such a comb filter, it is difficult to achieve, prevention of some color information from mingling with the luminance signal. In other words, for the reason that the subtraction ratio of the subtractors 6 and 7, the transmission band characteristic of the band pass filter 4, and the discrepancy between the delay characteristics of the delay circuits 5 and 41 vary in manufacture from item to item, the possibility of failure of isolating the luminance signal perfectly from the color signal is not negligible. That part of the color information mingled when in recording produces a beat when in reproduction.